1998 Ford F-150. When we put the truck in gear the speedometer jumps up to 40 even if barely moving. OUt on the highway it will raise up to normal speeds. But below 40 I am having to guess at the speed.

Also I am having a bit of a rough idle problem. Ran Sea Foam through the gas, brake booster line and oil last week and that has helped correct a miss in the engine. But still idles a bit rough. Any suggestions on correcting the above problems?

 

Try cleaning or replacing the IAC